Sound reproducing system



June 2, 1931. A RADFQRD ET AL 7 1 ,807,602

SOUND REPRODUCING SYSTEM I Filed Aug. 29. 1929 MW W 1 WW Patented June2, 1931 UNITED STATES PATENT OFFICE ARTHUR STANLEY RADFORD, OI SOUTHALL,AND MICHAEL BOWMAN-MANIFOLD, OF

HAYES, ENGLAND, ASSIGNORS TO VICTOR TALKING MACHINE COMPANY, A COR-PORATION OF NEW JERSEY sormn nnrnonucme svs'rnn Application filed August29, 1929, Serial No. 389,280, and in Great Britain September 27, 1928.

The present invention is concerned with improvements in amplifiers foruse with light sensitive devices such for example, as photoelectriccells. One of the objects of the invention is to eliminate or reducenoises caused by the variation in the intensity oi the light source withwhich the photo-electric cell is used, for example, due to the heatingof the filament of the light source with alternating current, or byvariation in the voltage of the anode supply battery. I

According to the present invention apparatus for the reproduction ofrecorded sound comprises two light sensitive devices and an amplifieradapted to amplify electrical oscillations derived from said devices andcorresponding to the difference between their respective illumination.

According to the present invention in the preferred form, each of saidlight sensitive devices is associated with the grid circuit of athermionic valve, one of said devices being illuminated from a lightsource by light modulated by a sound record and the other of saiddevices being illuminated from said source by light which is notmodulated by said sound record.

The invention is illustrated, by way of example, in the accompanyingdrawing.

Referring to the drawing, the anodes of two thermionic valves V and Vare connected through separate high resistances 1, 2 to the positiveterminal of a battery 3 the negative terminal of which is connected tothe filaments of the valves. The primary winding of a transformer 4 isconnected between the two anodes and the secondary winding of thistransformer constitutes the output of the amplifier. The filaments areheated in parallel from a low tension battery 5, a variable resistance 6being included in the filament heating circuit.

Each of the grids of the valves V and V is connected to the filamentcircuit throu h a grid leak 7 and in shunt with each of the grid leaksis a photo-electric cell 8, 9 in series with a battery 10. The twophotoelectric cells are arranged with their anodes connected to thegrids of the valves.

The cell 8 receives the light from the source said devices only.

11 after the light has passed through an optical system 12, a slitdevice 13 and a sound record film 14. The cell 9 receives the lightdirect from the source 11. The source 11 may be in the form of afilament lamp heated by alternating current. The film 14 is caused totravel past the slit device 13 producing variations in the illuminationof the cell corresponding to the sound markings on the film. Since theother photo-electric cell is illuminated directly by the light sourcethe sound record does not afi'ect it. When the adjustment is correct itwill be found that hum due to the alternating current in filaments ofthe light source is not transmitted to the output circuit butdifferences in illumination of the two cells 8, 9 is transmitted in theform of electrical oscillations to the output circuit.

In a modification of a circuit above de scribed, high tension issupplied to the anodes of the two valves V and V through the primarywinding of the output transformer 4 the centre point of this windinbeing connected to the positive terminal -0 the battery and theresistances 1 and 2 can then be omitted.

We claim: 1. In apparatus for the reproduction of recorded sound, thecombination of two thermionic valve amplifiers arranged in oppositionand having a common'output, two light sensitive devices one connectedacross the input of each amplifier, a common light source for saiddevices, and means for modulating the light passing from said source toone of 2. In apparatus for the reproduction of recorded sound, thecombination of two thermionic valve amplifiers having input and outputcircuits and arranged in opposition, a resistance in each of said outputcircuits, a resistance in each of said input circuits, a translatingdevice connected to both output circuits, two light sensitive devicesone connected in each input circuit, acommon light source for said lightsensitive devices, and means for modulating the light passing from saidsource to one only of said light sensitive devices.

3. In apparatus for the reproduction of recorded sound, the combinationof two thermi-- onic valve amplifiers, a common source of anodepotential for saidQamplifiers a resistance connected between said sourceandeach of said amplifiers, a translating device connected across saidresistances, a resistance connected across the input of each of saidamplifiers, a light sensitive device in parallel with each inputresistance, a common light source for said light sensitive devices,means for modulating the light passing from said source to one only ofsaid devices.

In testimony whereof we have signed our names to this specification.

ARTHUR STANLEY RADFORD.

MICHAEL BOWMAN-MANIFOLD.

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